Solid carbonaceous subterranean formations contain significant quantities of natural gas. This natural gas is composed primarily of methane. The majority of the methane is sorbed onto the carbonaceous matrix of the formation and must be desorbed from the matrix and transferred to a wellbore in order to be recovered. The rate of recovery at the wellbore typically depends on the gas flow rate through the solid carbonaceous subterranean formation. The gas flow rate through a solid carbonaceous subterranean formation is affected by many factors including the matrix porosity of the formation, the system of fractures within the formation, and the stress within the carbonaceous matrix which comprises the solid carbonaceous subterranean formation.
An unstimulated solid carbonaceous subterranean formation has a natural system of fractures, the smaller and most common ones being referred to as "cleats" or collectively as a "cleat system". To reach the wellbore, the methane must desorb from a sorption site within the matrix and diffuse through the matrix to the cleat system. The gas then passes through the cleat system to the wellbore.
The cleat system communicating with a production well often does not provide for an acceptable methane recovery rate. In general, solid carbonaceous subterranean formations require stimulation to enhance the recovery of methane from the formation. Various techniques have been developed to stimulate solid carbonaceous subterranean formations and thereby enhance the rate of recovery of methane from these formations. These techniques typically attempt to enhance the desorption of methane from the carbonaceous matrix of the formation and/or to enhance the permeability of the formation.
One example of a technique for stimulating the production of methane from a solid carbonaceous subterranean formation is to complete the production wellbore with an open-hole cavity. First, a wellbore is drilled to a location above the solid carbonaceous subterranean formation. The wellbore is cased and the casing is cemented in place using a conventional drilling rig. A modified drilling rig is then used to drill an "open-hole" interval within the formation. An open-hole interval is an interval within the solid carbonaceous subterranean formation which has no casing set. A metal liner, which has holes, may be placed in the open-hole interval if desired. The open-hole interval can be completed by various methods. One method utilizes an injection/blowdown cycle to create a cavity within the open-hole interval. In this method, air is injected into the open-hole interval and then released rapidly through a surface valve. The procedure is repeated until a suitable cavity has been created. During the procedure, a small amount of water can be added to selected air injections to reduce-the potential for spontaneous combustion of the carbonaceous material of the formation.
A limitation of this technique is that its effectiveness in efficiently increasing methane recovery is mainly limited to formations where formation pressure and permeability are high, such as in the "fairway" zone of the San Juan Basin located in northern New Mexico and southwestern Colorado.
Gel and foam fracture treatments are examples of other types of stimulation techniques which have been used to increase the methane recovery rate from a formation. These stimulations typically are conducted in formations where the region of the wellbore penetrating the solid carbonaceous subterranean formation is completed with a cased hole technique, a so-called cased-hole interval. With a cased-hole interval, the region of the wellbore penetrating the solid carbonaceous subterranean formation is cased and the casing is cemented in place using conventional techniques. The stimulations use of a high viscosity fluid, such as gels or foams, will assist in transporting proppant, if utilized, into the formation. The proppant is injected into the formation through perforations formed in the casing adjacent the formation. The high viscosity fluids are injected at pressures above the parting pressure of the formation. The injection of fluid at pressures above parting pressure induces a new dominant fracture, or fracture system, which is intended to better connect the formation to a production well. The injection is continued for the desired length of time and then ceased. The fluid preferably carries a proppant to hold the fractures open once the injection pressure is released. In general, the injection of the fluid is not repeated.
Unfortunately, gel and foam fracture techniques often result in damage to the formation due to the interactions between the high viscosity fluid and the formation matrix. Additionally, conventional fracture techniques mainly create tensile fractures within the formation and do not cause substantial shear failure within the formation. It is believed by the inventors of the present invention that the creation of shear failure within the formation is important for enhancing the recovery of methane from a formation. Because conventional fracture techniques do not cause significant shear failure within the formation, they do not significantly reduce the stress within the formation. In fact, if proppants are utilized with conventional fracture techniques, the proppants often increase the stress within the carbonaceous matrix. This increase in stress can reduce the recovery of methane from the formation by compressing the cleats and reducing the permeability of the formation.
A third stimulation technique which has been utilized to enhance the methane recovery rate from a formation is water fracture treatments. Like gel fracture treatments, this technique is typically utilized in formations in which the wellbore interval penetrating the formation is completed with a cased-hole technique. The treatments typically are conducted through perforations in the casing adjacent the formation. The water is injected at a pressure above the formation parting pressure of the formation, inducing a new dominant fracture, or fracture system, which is intended to better connect the formation to a production well. The technique optionally utilizes proppants to hold the fractures open. Like gel fracture treatments, conventional water fracture treatments generally do not cause substantial shear failure within the formation.
Puri et al., U.S. Pat. No. 5,014,788, discloses a method for increasing the permeability of a coal seam by introducing a fluid into the coal seam which causes the coal to swell. The pressurized fluid is maintained within the seam to enhance the contact between the fluid and the coal seam. The pressure within the seam is relieved by allowing the fluid to flow out the wellbore prior to the pressure within the coal seam decreasing to a stabilized pressure. The method of the patent is intended to increase the permeability of a coal seam located near the wellbore. The patent teaches that the procedure may be repeated but it does not disclose how many times to repeat the procedure or how to determine how many repetitions are to be performed.
What is needed is a method for stimulating a solid carbonaceous subterranean formation to increase the rate of methane recovery from the formation which enables various fluids to be used to stimulate the formation while minimizing the damage to the permeability of the formation.